The mixed function oxidase is responsible for the oxidative modification of a wide variety of naturally occurring steroids and fatty acids as well as a host of drugs, insecticides, petroleum products and carcinogens. Many foreign compounds are detoxified by the mixed function oxidase, while in certain other cases metabolism of these agents by this system results in their activation to cytotoxic, mutagenic or carcinogenic products. Cytochrome P-450LM is the terminal oxidase of the mixed function oxidase system in which completely resides the substrate specificity and catalytic activity for oxidative modification. Recently cytochrome P-450LM has shown to consist of distinct forms. These enzymes exhibit both regio-and steroselectivities both toward the substrate and with respect to product formation. Although much data is available on the metabolism of a wide variety of compounds by P-450LM, information is lacking as to the molecular basis of P-450LM substrate specificity and regio- and stereoselectivity. Thus, the present investigation will not only provide a substantial insight into the basis of P-450LM substrate specificity, possibly resulting in a technique which is capable of predicting sites of oxidative modification, it will also provide a great deal of information as to the effects of the mixed function oxidase components and related enzymes upon the substrate-P-450LM interaction. Since a number of compounds may be bioactivated via the mixed function oxidase to mutagenic, cytotoxic or carcinogenic products, the ability to understand the molecular basis of substrate-P-450LM substrate specificity (to include regio- and stereoselectivity) and which form of P450LM may ultimately be responsible for such bioactivation would be important in estimating the concentration and nature of the toxic or reactive product formation.